Note: Descriptions are shown in the official language in which they were submitted.
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B~CKGROUND OF THE INVENTION
The present invention relates in general to ground
based reception of orbiting satellite relayed television signals,
and in particular, to an improved self-contained tuner-
demodulator subsystem for television satellite receivers which
serves to select or "tune into" a particular television channel.
The present tuner-demodulator device incorporates all of the
necessary circuitry between the first intermediate frequency band
(950-1750 MHz) presented to the satellite receiver dual input,
and the demodulated based band (O-lOMHz). While satellite
television tuner-demodulator systems exist for the demodulation,
manipulation and control of television signals received by ground
based satellite antenna, and are capable of controlling and
otherwise manipulating same towards use with a television monitor
or display, few, if any such devices have addressed themselves or
otherwise permitted flexible use with an alternative number of
television system signals particularly in view of the growing
number of commercial and consumer television system applications
including, direct broadcast, SMATV, TVRO and MAC.
There are at present, and it can be expected in the
future that there will be an increasing number of strong input
signals as more and more countries either start or expand their
DBS transmissions, and accordingly, an important requirement of a
tuner-demodulator device is to possess low intermodulation
distortion.
Accordingly, the present invention has as one of its
objects, the provision of an improved tuner-demodulator device -
which possesses low inter-modulation distortion.
It is further an object of the present invention to
provide a tuner-demodulator which posse~ses minimum
intermodulation distortion in the various stages thereof and
accordingly possess excellent performance of the first
intermediate frequency and mixer stages so as to no~ be
necessary to include gain control in the first intermediate
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frequency stage.
Another object of the present invention is to provide
for matching the input of the tuner-demodulator to the 75 ohm
impedance of coaxial cable and to further compensate for and
otherwise control image rejection, second intermediate frequency
rejection, gain and the suppression of oscillator radiation to
other indoor units connected and parallel with the present
device.
These and other objects of the invention will become
apparent in light of the present specification, drawings and
claims.
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SUMMARY OF THE INVENTION
The present invention comprises a self-contained tuner-
demodulator device for use with a ground based satellite signal
reception antenna and a low noise block down converter (LNB)
which is capable of capturing satellite relayed television
signals and providing a block down converted radio frequency
signal to a satellite receiver unit toward the viewing of
television images upon a television display monitor. The low
noise block down converter (LNB) is installed at the focus of the
dish antenna to convert the 4 GHz satellite signal to a 1 GHz
first intermediate frequency signal.
The tuner demodulator device comprises an input stage
for operably and electrically connecting the tuner-demodulator
device to the LNB. A first radio frequency amplifier operably
and electrically connected to the input stage amplifies the LNB
output signal which comprises a low power high frequency type
signal.
A local oscillator, preferably a voltage controlled
oscillator, is provided for converting the high frequency LNB
output signal to an intermediate frequency signal. Associated
with the local oscillator is a buffer operably and electrically
associated therewith for isolating the local oscillator to
prevent unwanted distortion of the LNB output signal.
An image tracking filter is operably and electrically
connected to the first radio frequency amplifier, the image
tracking filter serving to select the desired channel to be
viewed on the television monitor. The image tracking filter
serves to convert the broad band high frequency LNB output signal
into a narrow band signal, the particular band corresponding to
the desired channel. A secondl radio frequency amplifier,
operably and electrically associated with the image tracking
filter, serves to amplify the narrower band LNB output signal
corresponding to the selected channel.
A mixer is operably and electrically connected to the
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local oscillator, the buffer and the second radio frequency
amplifier for combining the LNB output signal and the output of
the oscillator towards the generation of a reduced frequency
signal corresponding to the selected channel, preferably in a
second intermediate frequency band distinct from the 1 GHz
intermediate frequency signal generated by the LNB. A first
intermediate frequency amplifier is electrically coupled to the
mixer and serves to amplify the second intermediate frequency
signal corresponding to the selected channel.
A first filter means operably and electrically
connected to the first intermediate frequency amplifier is
provided, together with a second intermediate frequency amplifier
coupled to the first intermediate frequency amplifier both of
which serve to amplify the signal corresponding to the selected
channel. The demodulator is operably and electrically connected
to the second intermediate frequency amplifier towards converting
the intermediate frequency signal corresponding to the selected
channel to a broad band signal suitable for use with a satellite
receiver unit and television display monitor.
In the preferred embodiment of the invention, the tuner
demodulator device further includes an input stage which
incorporates a vertical/horizontal switch serving to permit
selection between a horizontally polarized and vertically
polarized input signal collected by the ground based sa~ellite
reception antenna and modified by the LNB. The tuner demodulator
further includes a prescaler for providing compatibility with
digital tuning systems incorporating computer interfaces towards
digital tuning and control of the tuning system.
A second filter means is preferably operably and
electrically associated with the mixer for suppressing the local
oscillator and first intermediate signals. The second filter may
be fabricated as a band pass filter configured to select a second
intermediate frequency of 479.5 ~z.
The preferred embodiment of the invention may further
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include a terrestrial interference filter having a 13 MHz pass
band for suppressing terrestrial noise and interference.
Preferably, the terrestrial interference f.ilter may be switchably
connectable to the tuner-demodulator device towards its optional
selective use. A jumper is operably and electrically connected
between the second intermediate frequency amplifier and the
demodulator wherein the jumper permits the interconnection of an
external circuit or component, such as filters and/or other
optional external demodulator devices. The jumper permits future
expandability and compatibility of the present tuner-demodulator
device with future television broadcast systems.
A detector means is preferably operably and
electrically connected to the second intermediate frequency
amplifier and the first intermediate frequency amplifier towards
providing automatic gain control of the first intermediate
frequency amplifier means. The detector serves to detect the
level of the signal output from the second intermediate frequency
amplifier and compares same to a present level. The detector
serves to automatically adjust the gain of the first intermediate
frequency amplifier towards maintaining the input level to the
demodulator at a constant level thereby obtaining optimum
performance of the device.
The image tracking filter preferably comprises a broad
bandwidth filter having a 150 MHz bandwidth at the lowest
frequency channel.
The demodulator may comprise a phase-locked loop
circuit having a phase detector, voltage controlled oscillator,
loop amplifier and output buffer stage towards the compensation
of differences between the voltage controlled oscillator
frequency and the input to the demodulator means. The foregoing
FM demodulator may of course be implemented with demodulators
other than the phase-locked loop circuit disclosed herein.
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i BRIEF DESCRIPTION OF THE DRAWINGS
Fig. la is a block diagram of the electrical components
of the tuner-demodulator device; including the V-H switch, first
and second radio frec~uency amplifiers, image tracking filter,
mixer, voltage controlled oscillator and prescaler;
Fig. lb is a block diagram of a portion of the tuner-
demodulator device, including the first and second intermediate
frec~uency amplifiers, terrestrial interference filter, and FM
demodulator;
Fig. 2a is a schematic circuit diagram showing the
Vertical-Horizontal (V/H) switch and the first radio frecluency
amplifier portions o~ the tuner-demodulator device;
Fig. ~b is a schematic circuit diagram showing the
image tracking filter and the second radio frec~uency amplifier
portions of the tuner-demodulator device;
Fig. 3a is a schematic circuit diagram showing the
mixer portion of the tuner-demodulator device;
Fig. 3b is a schematic circuit diagram showing the
voltage controlled oscillator portion of the tuner-demodulator
device;
Fig. 4a is a schematic circuit diagram showing the
first intermediate frequency amplifier and bandpass filter
portions of the tuner-demodulator device;
Fig. 4b is a schematic circuit diagram showng the
terrestrial interference filter and second intermediate amplifier
portions of the tuner-demodulator device;
Fig. 5 is a schematic circuit diagram of the jumper
portion of the tuner-demodualtor device;
Fig. 6a is a schematic circuit diagram showing the
video amplifier portion of the tuner-demodulator device;
Fig. 6b is a schematic circuit diagram showing the
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signal strength detector portion of` the tuner-demodulator device;
Fig. 6c is a schematic c:ircuit diagram showing the FM
demodulator portion of the tuner-de!modulator device;
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Fig. 7 is a diagram indicating the relationships of the
other drawings to each other.
2~)C)(~ 7
DETAILED DESCRIPTION OF THE DRAWINGS
While this invention is susceptible of embodiment in
many different forms there is shown in the drawings and will
herein be described in detail, one specific embodiment, with the
understanding that the present disclosure is to be considered as
an exemplification of the principles of the invention and is not
intended to limit the invention to the embodiment illustrated.
Figs. la and lb of the drawings comprise a block
diagram of the electrical submodule portions of the tuner-
demodulator device 10 forming the present invention. In its
typically contemplated use, the present tuner-demodulator device
is connected to and designed to accept a high frequency
television signal input collected from a ground based satellite
receiving antenna, such as a satellite dish and low noise block-
down converter (LNB). The satellite dish accepts and receives a
combined audio-video signal from an orbiting communication
satellite and in turn conducts the received signal through the
LNB to the tuner-demodulator towards the display of a television
image upon a television monitor. The input portion of the device
is formed by the connection of line 11 and line 12 to vertical-
horizontal switch 13 (V/H switch) each of which conducts the LNB
output signal from the satellite receiving dish and LNB to the
tuner-demodulator device. Vertical/horizontal switch 13 serves
to permit selection between the two inputs to the tuner-
demodulator device towards accepting and/or compensating for
systems which use either a vertically polarized or horizontally
polarized signals, as for example those systems using two inputs
to permit selection between a 4 GHz signal in the C band or a 12
GHz signal in the KU `band. V-H switch 13 is in turn shown
connected to first radio frequency (RF) ampli~ier 15 which serves
to amplify the high radio frequency-low power signal received by
~ -~ satellite dish antenna and LNIB.
z~C~(~167
First RF amplifier 15 is electronically connected to
image traeking filter 16. Image tracking filter 16 is shown
having a control input 17 designated "Vt". Vt control input 17
i5 adjustable and serves to designate the particular desired
frequency and thus the particular channel of the television
signal sought to be viewed. Vt control input 17 is further shown
conneeted to voltage eontrolled oseillator 21 by eonnection lB.
Image traeking filter 16 is thus shown connected to a second RF
amplifier 19 which similarly amplifies the high frequency-low
power signal which now corresponds almost solely to the
television channel sought to be viewed.
The output of the seeond RF amplifier 19 is connected
to mixer 20. Mixer 20 serves to combine the output of the seeond
RF amplifier 19 with the output of a voltage controlled
oscillator (VCO) 21. The Vt control input 17 is connected to
voltage controlled oseillator 21 whieh is in turn controlled in
part by the output of prescaler 22 which in conjunetion with
digital eireuitry not shown serves to fine tune Vt eontrol input
17. The output of mixer 20 (see Fig. lb) is an intermediate
frequeney (IF) signal whieh is eonneeted to bandpass filter 26
the output of whieh passes through terrestrial interference
filter 27 or may bypass filter 27 by switches 29 and 30 which
conduct the intermediate signal over bypass line 28. Thereafter
the signal is condueted to seeond intermediate frequeney
amplifier 32 the output of whieh is eonneeted by input 33 to
jumper 34. The output 35 of jumper 34 is thereafter conducted to
demodulator 36 and detector 37 by way of line 42. Jumper 34
permits the present device to be expanded by allowing the signal
to be eonneeted to a separate filter system not a part of the
present design. Demodulator 36 converts the intermediate
frequeney signal to a base band signal. With the external video
demphasis/clamping and audio subcarrier demodulation circuits,
the base band signal is capable of generating a video image and
audio sound upon a television monitor.
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Detector 37 has its output 39 connected to flrst
intermediate frequency amplifier to provide for automatic gain
control of the amplifier 25 by detecting the signal strength
present in output 35 and comparing it to a preselected value and
adjusting the gain of first intermediate frequency amplifier 25
accordingly.
In the embodiment illustrated, the output 38 of
demodulator 36 is connected to video amplifier 43 which provides
an output 44 capable of connection to a television monitor
through a de-emphasis and clamping circuit.
In the preferred embodiment of the invention, the
tuner-demodulator device is fabrica~ed upon a double-sided copper
epoxy printer circuit board having an Er equal to 4.5 and a
thickness of 1.6 mm. One side of the printed circuit board
serves as a ground plane and both sides are interconnected at
several places to improve the RF quality of the ground plane.
Except for the required coils, all of the high frequency
components, including transistors and varicaps, are preferably
surface mounted devices which minimize parasitic effects and
reduce the size of the device.
Figs. 2a and 2b of the drawings show the schematic
circuit diagram of V/H switch 13 (including inputs 11 and 12),
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first RF amplifier 15, image tracking filter 16 and second RF
amplifier 19. In Fig. 2a, V/H switch is shown being activatable
by V/H input 14 which actuates a relay 50 to switch between -~
inputs 11 and 12 respectively. The output of V/H switch 13 is ~-
connected to first RF amplifier 15 by line 51. First RF
amplifier 15 is a broadband amplifier based upon a 2SC4093
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transistor 52. The amplifier 15 utilizing the microstrip
matching network builds a broadband amplifier covering the
frequency range from 0.95 GHz to 1.75 GHz wherein a power gain of
6 dB may be obtained. Since emitter bonding wires may induce
unwanted series feedback in the grounded emitter configuration, a
dual emitter connection in the SOT-143 package is utilized.
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Moreover, a bias of 15 mA collector current in the transistor
gives low intermodulation and optimum gain. The output of the
first RF amplifier 15 is connected to the imaging tracking filter
16 by line 53 as shown in Fig. 2b. Unless otherwise specified on
drawings, all capacitor values are in Farads, 50 volt rated,
resistors are in Ohms, 1/10 W rated, inductors are in Henry, 1/10
W rated. MLIN, MLSC and MCLIN designations refer to microstrip
line elements constructed directly in the printed circuit board.
Imaging tracking filter 16 is a bandpass filter. Although fixed
filters are suitable for suppressinq image and second IF
breakthrough, they cannot suppress oscillator radiation to the
antenna input connector due to the oscillator radiating at a
frequency 480 MHz higher than the received channel which is in
the incoming band when tuned to lower frequency satellite
channels. Consequently, a tracked filter is required to isolate
the local oscillator signal from the LNB input. To avoid
oscillator tuning and tracking problems a broad bandwidth filter,
150 MHz bandwidth at the lowest frec~uency channel, is chosen.
The filter is tuned by two ISV186 varicap diodes which possess an
extremely low capacitance, typically in the range of 0.66 to 0.82
pF. Preferably, the maximum/minimum capacitance ration is
approximately 5.2 for a tuning voltage in the range of 2 to 20
volts.
The configuration is a conventional double tuned
circuit. However for the high frequency application, the
transformer is replaced by two coupled microstrip lines 54 and
55. Another two microstriplines 56 and 57 are used to match the
double tuned filter to the transistors 52 and 59 of the first and
second RF amplifiers. The insertion loss of image tracking
filter 16 is approximately 3 dB.
The output of image tracking filter 16 is connected to
second RF amplifier 19 by line 58 which is a broadband amplifier
substantially identical to first RF amplifier 15 and likewise
incorporates a 2SC4093 transistor 59. The output 60 of second RF
amplifier 19 is connected to mixer 20 shown in Fig. 3a.
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Figs. 3a and 3b of the drawings comprise a schematic
circuit diagram of voltage controlled oscillator 21 and mixer 20
portions of the tuner-demodulator device 10. Voltage controlled
oscillator 21 (Fig. 3b) incorporates a local oscillator and a
buffer stage. For a single band tuner with a second IF of ~79.5
MHz, the local oscillator frequency must be in the range of
1429.5 MHz to 2229.5 MHz. The local oscillator shown is a
negative impedance oscillator incorporating a 2SC3583 transistor
61 having a collector-emitter voltage of 6 volts and a collector
current of 8 mA. The local oscillator is tuned by two ISV186
varicap diodes 62 with resonant circuit inductance implemented by
copper strips 62a. The local oscillator is isolated from the
mixer 20 by a 2SC3583 buffer stage 63 which prevents distortion
and pulling of the oscillator by strong first IF signals applied
at the mixer 20.
For compatibility with digital tuning systems, a
prescaler is included in the tuner-demodulator device with input
buffer 64. The prescaler divides the local oscillator frequency
by 128 to lower the frequency to permit it to be read by an
external microprocessor and external phase locked loop circuit
thereby serving to control input Vt 17~ Fine tuning of the
selected channel is thereby possible.
Mixer 20 is based upon a NE41137 dual gate MESFET 66.
Local oscillator isolation is inherent in ~he design as the
output of local oscillator 21 fed to mixer 20 by line 65 and the
first IF signal are injected to G2 and Gl respectively of MESFET
66. Filtering of the local oscillator and first IF signals is
also done by the complex matching of gates. The NE41137 MESFET
66 in a common source configuration gives excellent
intermodulating discrimination and a flat 4 dB conversion gain.
The output of mixer 20 is connlected to a bandpass filter to
select a second IF signal of 479.5 MHz, and suppress the local
oscillator and first IF signal. 'rhis second IF frequency reduces
number of first IF filters needed. Note also that all image
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frequencies are outside of the incoming down converted band
frequencies.
Figs. 4a and ~b of the drawings show the first
intermediate frequency amplifier 25, filter 26, terrestrial
filter 27 and second intermediate frequency amplifier 32. The
output on line 67 of mixer 20 is connected to the first
intermediate amplifier 25 (Fig. 4a) which is a three stage
amplifier incorporating transistors 68, 69 and 70 which are BF990
dual gate MOSFET, BFR92A and BFR92A types respectively. This
amplifier possesses a gain of 44 dB. Gain control is implemented
to achieve the required -5dB signal level at the input of the
phase-locked loop ~PLL) demodulator 36 while providing a low
intermodulation in the second IF stages. A gain control range of
40 dB is achieved which is sufficient to handle a variety of
specific receiving conditions, such as different antenna
diameters, divergent satellite power levels and cable losses. -~
Currents in the different stages have been chosen to give both
low power consumption and minimum intermodulation.
To suppress adjacent channel interference, a ceramic
printed filter 71 with a center frequency of 479.5 MHæ and a 27
MHz passband is utilized. ~ -
In addition, (as shown in Fig. 4b) a switchable
terrestrial interference filter 27 having a 13 MHz passband is
provided to suppress terrestrial interference and noise created
by local television stations present in some geographic regions.
Inductors 72 and 73 create the required notch filter.
The output on line 74 of terrestrial filter 27 is
connected to the second intermediate amplifier 32 which is a two
stage amplifier incorporating transistors 75 and 76 which are
both BFR92A type. Transistors 70 and 75 are additionally 50 ohm
matched transistors for proper driving of the filters
therebetween.
The output 33 of second intermediate frequency
amplifier 32 is connected to jumper 34, Fig. 5, the output 35 of
which is connected to demodulator 36 and by line 42 is connected
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to detector 37 for automatic gain control of first IF amplifier
25. Jumper 34 is provided to permit the connection of an
external filter for future use. Absent use of an optional
external filter, the output of second intermediate frequency
amplifier 32 is connected to demodulator 36 and detector 37 as
above described.
Figs. 6a, 6b and 6c of the drawings show the schematic
circuit diagrams for demodulator 36, detector 37 and video
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amplifier 43. Output 35 from jumper 34 is connected to
demodulator 36 and to detector 37 by line 42. FM demodulator 36
(Fig. 6c) converts the incoming frequency modulated signal to a
baseband signal. While frequency demodulation can be
accomplished using a number of methods, the present tuner~
demodulator device utilizes a phase-locked loop method, though
such other methods are contemplated and deemed within the scope
of the invention. The phase-locked loop method has the advantage
of being able to flexibly handle a variety of television formats
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presently available, as well as new formats such as HDTV (high `
definition television) or MAC TV transmissions. Phase-locked
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loop also allows for extension of the carrier to noise (C/N)
ratio threshold which prevents annoying deterioration in picture
quality at low C/N ratios. Other qualities of the present design
are high signal output and ease of adjustment.
The phase-locked loop FM demodulator 36 (Fig. 6c)
consists of a phase detector, voltage controlled oscillator, loop
amplifier and output buffer stage. When the phase-locked loop is
locked, the instantaneous voltage controlled oscillator frequency
will always try to follow the IF input frequency, and the phase
detector outputs an error voltage if the two frequencies are
different. This error voltage is integrated by a lag/lead type
loop filter and corrects the voltage controlled oscillator
frequency through a DC amplifier. The value of the loop filter
components depend on both the incoming satellite signal
modulation and the type of television signal to be processed.
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The components are readily common and chosen to give optimum
threshold extension. As shown in Fig. 6a, the demodulated signal
is buffered by three transistors 77, 78 and 79 so that the output
loading and the demodulator are fully isolated.
Detector 37 (Fig. 6b) serves to detect any difference
between the IF signal strength and a preset value by means of
comparitors 80 and 81 both of which are LM358N type and diode 82
a ISS106 type. Detector 37 functions as an automatic gain
control by adjusting the gain of first in~ermediate frequency
amplifier 25 based upon the outcome of the comparison made by
detector 37. The foregoing serves to keep a constant input
signal level at the demodulator to thereby maintaining optimum
performance.
Output 44 thus comprises a baseband raw video signal
suitable for use by accompanying system circuitry.
The foregoing description and drawings merely explain
and illustrate the invention and the invention in not limited
thereto, except insofar as the amended claims are so limited as
those skilled in the art who have the disclosure before them will
be able to make modifications and variations therein without
departing from the scope of the invention.
